In fifteen years, astronomers have detected just over 500 planets orbiting other stars. Now, after less than two years in orbit, the Kepler telescope has more than tripled that figure, discovering more than 1,200 possible new worlds. Even better, at least 54 of them are in their star's habitable zone, and five of these are Earth-sized planets. But here's the best part - Kepler only looked at one tiny corner of our galaxy. We really might be surrounded by millions of Earth-like planets after all.

Turns out yesterday's story about a tightly packed solar system was just a tease for the really big news. After just 23 months in orbit, NASA's Kepler telescope has discover 1,235 potential planets orbiting around the 156,000 stars it's been charged with observing. It should be stressed that these are still just possible planets, but in all likelihood the vast majority of these observations will stand up, so we're still probably looking at at least a thousand new exoplanets.

So what sorts of planets has Kepler found? There appear to be 68 Earth-sized planets, 288 rocky planets that are considerably bigger and known as Super-Earths, 662 relatively small gas giants around the size of Neptune, 165 Jupiter-sized planets, and 19 that are even bigger than Jupiter. The discovery of nearly 70 Earth-like planets is particularly exciting, considering we had only detected a handful of them before this.

Before we get too excited, let's look at the data from a different angle. When we talk about Earth-like planets, size is only part of the equation - the real issue is whether they are capable of supporting life, and that means they have to be in their stars' habitable zones. 54 of Kepler's candidate planets fall within this zone, although most of these are gas giants or huge Super-Earths where the gravity is probably too strong to make life all that likely. But five of these planets in the habitable zone are roughly Earth-sized, and at least one is very close to the same size of our planet. That could be a big deal.

Now, we do need to be cautious about this. The discovery of five Earth-like planets in their stars' habitable zones is hugely exciting, and it certainly raises the odds considerably that there are other planets out there that have water and life just like Earth. But we shouldn't be too hasty to celebrate - after all, we've already known about an Earth-like planet inside its star's habitable zone for eons, as Mars actually does just about fit both those definitions. Of course, a few ancient microbes aside, there doesn't appear to be much by way of life on Mars.

There's any number of reasons why an Earth-sized planet in the habitable zone might not end up supporting life, and there's no guarantee any of these five candidates is even remotely Earth-like where it really counts.

But we need to look at this from a larger perspective. Kepler looked at 156,000 stars, sure, but that's nothing in galactic terms. The telescope only scanned about one millionth of the entire Milky Way, and if these numbers are at all representative, then we might be looking at five million Earth-sized, potentially habitable planets scattered throughout the galaxy. The odds of habitability might not be 1 in 5, but they're almost certainly a lot better than 1 in 5 million.

And besides, there's still the gas giants in the habitable zone. These planets are capable of supporting very large moons that in turn could be able to support life. Indeed, if you swapped Earth with either Jupiter or Saturn, their frozen moons would thaw into watery paradises that could quite possibly support life. Again, it can't be considered likely that any of the planets Kepler discovered fit this description, but spread across the entire Milky Way? It's suddenly looking very good.

So what are we left with? I think Phil Plait over at Bad Astronomy says it best:

During the Kepler press conference, planetary astronomer Debra Fischer called this "an incredible, historic moment." I agree! For years we've been making progress toward finding another blue-green world around another star, and this news means we've taken a really big stride in that direction.

For the first time in human history, we can look out into the night sky and actually and realistically and scientifically consider the presence of other Earths out there.

For more on the science of how Kepler found all these planets, as well as some very persuasive reasons why the million Earth-like planets figure might actually be too low, we definitely recommend you check out the post at Bad Astronomy.